1. Field of the Invention
The present invention relates to a communication system, and in particular to a transmission apparatus for a CDMA communication system.
2. Description of the Conventional Art
Generally, in the cellular communication system, in order to decrease the effects of any interference, a spectrum spreading technique capable of more widely spreading a carrier spectrum compared to an information ratio is used. As a CDMA spectrum spreading technique, there are known a direct spreading method (DS: Direct Sequence) and a frequency hopping (FH) method.
The DS-CDMA method is directed to spreading an information signal into a designated spectrum region by modulating the information signal using an ideally random spreading code. Actually, as the above-described spreading code, the sequence of the psuedorandom numbers which is a signal stream repeating at a long period is used.
In addition, a receiver reversely spreads the spread signal using a reverse spreading code which is the same as the spreading code.
The above-described operation will be explained with reference to the following equations.
The spreading operation at the side of a transmissiol may be expressed as follows.
xe2x80x83y(t)=a(t)xc2x7c(t)xe2x80x83xe2x80x83(1)
where the audio data which is first modulated to a PCM signal is a(t), the PN code is c(t), and the wave form transmitted is y(t).
Assuming that the signal at the side of the transmission is not decreased, and there is not any interference or noise, the wave form at the side of the receiving is y(t), and the reverse spreading output z(t) may be expressed as follows because the code same as the spreading code c(t) is multiplied.
z(t)=y(t)xc2x7c(t)=a(t)xc2x7{c(t)}2xe2x80x83xe2x80x83(2)
Here, since c(t) is the sequence of the random numbers which is defined as +1 or xe2x88x921. therefore, when squaring c(t), a result of the computation is 1. Therefore, z(t) may be expressed as follows.
z(t)=a(t)xe2x80x83xe2x80x83(3)
Therefore, it is possible to recover the original data a(t) based on the reverse spreading process. Here, at the side of the receiving, it is needed to generate the wave form which is the same as the side of the transmission at the same time. The PN code is preferably a random number. If the PN code is the random number, since it is impossible to represent the condition that the sequences of the random numbers at the sides of the transmission and receiving are absolutely identical, the sequence of the psuedorandom numbers is used.
As described above, the DS-CDMA communication apparatus provided a communication service to a plurality of subscribers through a transceiver base station in a neighboring region. Since one base station is designed to provide a communication service to a plurality of communication apparatuses at the same time in the identical regions, a plurality of CDMA signals should be transmitted/received at the same time.
In order to effectively performed the above-described operation, a plurality of transceivers and antenna may be provided to one base station. However, this method is too expensive. Therefore, as a more economical method, there is a method capable of performing a signal transmitting/receiving operation using a transceiver capable of transmitting/receiving a plurality of signals through one antenna.
In order to implement a transmission/receiving operation of a plurality of CDMA signals through a common antenna, the outputs of each CDMA channel are combined for thereby amplifying the outputs and then transmitting the same through a transmission antenna.
When transmitting a plurality of CDMA signals, a HPA (High Power Amplifier) capable of computing the peak power, and amplifying the peak power at a predetermined amplification ratio in the linear amplification region of the amplifier is used.
The HPA is an amplifier capable of outputting the same output as a value obtained by multiplying the average power used for transmitting the data based on one CDMA channel by the PAR (Peak to Average Ratio) based on the average ratio.
For example, when transmitting the signals of 30 channels based on 1 Watt per channel, the HPA amplifier having the peak power (PP) computed based on the following Equation 4 is needed.
PP=CP*CH*PAR=AP*PAR=1 Watt*30Ch.*30=900 Wattxe2x80x83xe2x80x83(4)
PP=Peak Power (Watt)
CP=Channel Power (Watt)
CH=The number of channels
AP=Average Power (Watt)
PAR=Peak to Average Ratio
Namely, the amplifier used for a transceiver of the CDMA base station is an amplifier having a peak power of 900 Watt which is the linear amplification region.
The operation that the transmission power of the signal transmitted from the conventional CDMA base station is controlled and then transmitted will be explained with reference to the accompanying drawings.
FIG. 1 is a block diagram illustrating a part of the transmission apparatus for a CDMA base station capable of transmitting a plurality of CDMA signals through one antenna, and FIGS. 2A and 2C are views illustrating an output level for explaining an output in accordance with the number of channels of the CDMA type.
As shown in FIG. 1, the conventional transmission apparatus of the base station transmission includes a MUX (Multiplexer) 10 for multiplexing the CDMA signals inputted through a plurality of channels CH.1 through CH.n, an up-converter 20 for combining the multiplexed signals and a clock signal generated by a local oscillator 30 and then converting the signals into a high frequency signal, a HPA (High Power Amplifier) 40 for amplifying the up-converted signals to a predetermined transmission power level, and a duplexer 50 for transmitting the amplified signal through an antenna or transmitting the signals received through the antenna to the side of the receiving apparatus.
The data transmission of the conventional CDMA base station will be explained. When the CDMA signals inputted through the channels CH.1 through CH.n are multiplexed by the multiplexer 10 for thereby outputting one signal, the up-converter 20 combines the multiplexed signal with a clock signal generated by the local oscillator 30 and converts the signal into a high frequency and amplifies the same for thereby driving the high power amplifier 40.
The high power amplifier 40 amplifies the inputted signal to a predetermined level in accordance with a predetermined amplification ratio. The thusly amplified signal is applied to the duplexer 60 and is outputted through the antenna.
Here, the output levels at the high power amplifier 50 will be explained in more detail with reference to FIG. 2.
First, as shown in FIG. 2A, when transmitting a signal at one channel needing 1 Watt as a final output, the values of the average power of 1 Watt based on one input channel, and the peak power 1 based on the average ratio are obtained. Therefore, the linear amplifier 50 having the peak power of 1 Watt is needed.
In another example, as shown in FIG. 2B, when the signals are inputted from two channels, the values of the average power of 2 Watt based on two channels, and the peak power 2 based on the average ratio are obtained. Therefore, the peak power is 4 Watt based on Equation 4.
As shown in FIG. 2C, when the signals are inputted from a N-number of channels, the values of the average power of N-Watt based on the N-number of channels, and the peak of N based on the average ratio are obtained. Therefore, the peak power of N2 watt is obtained based on Equation 4.
In other words, the HPA 50 having a peak power as much as the value obtained by multiplying the average power based on a plurality of channels by the PAR based on the average ratio is needed at the final output terminal of the CDMA base station transmission apparatus.
However, the high power amplifier 50 is expensive. As the power of the same is increased, the price of the same is increased.
In addition, a lot heat is generated from the high power amplifier, a heat radiating apparatus having a high capacity is additionally used, so that the size of the entire transceiver of the CDMA base station using the high power amplifier 50 is increased, and the power consumption of the high power amplifier 50 is increased.
Accordingly, it is an object of the present invention to provide a transmission apparatus for a CDMA communication system which is capable of decreasing an input voltage inputted into a high output amplifier, being used for a plurality of subscriber channels and using a low price high output amplifier.
To achieve the above object, there is provided a CDMA communication system which includes a multiplexer for multiplexing CDMA signals inputted through a plurality of channels, a voltage limiter for limiting a voltage of the multiplexed signal to a predetermined voltage level, an up-converter for up-converting the signal inputted from the voltage limiter into a high frequency signal, and a high power amplifier for amplifying the up-converted signal to a predetermined power level.
Additional advantages, objects and features of the invention will become more apparent from the description which follows.